It trashed Olivia Newton-John’s film career, halted the movie-musical revival, and was so critically reviled it led to the creation of the Razzies. How, then, could the stage version of hubristic 1980 flop Xanadu become a 2007 Broadway hit? The answer, as illustrated by Paul Warwick Griffin’s sublimely silly Southwark Playhouse production, is to laugh at itself first.